It is known that, in nuclear power stations, and particularly in so-called pressurized water power stations, the water heated in the nuclear reactor is used to produce steam in heat exchangers termed steam generators, which have a bank of tubes through which passes the primary hot water circuit, positioned in a vessel in which the water of the secondary circuit is changed into steam.
In state of the art installations, the tubes of the primary circuit each form a U whose ends are fixed on a tube sheet provided with two series of orifices corresponding respectively to the inlet and outlet for the primary fluid.
The water box used to supply and remove the primary fluid is bounded at its upper part by the tube sheet on which a hemispherical base is fixed, divided into two parts by a semi-circular partition plate positioned in a diametrical plane. The water box is thus divided into two chambers, respectively for supply and evacuation and the hemispherical base is therefore provided with connection pipes, respectively for inlet and outlet conduits for the primary fluid. In addition, the hemispherical base is also provided with man-holes allowing access to each of the two chambers of the water box.
To date, it has seemed necessary to weld the partition plate onto the tube sheet along its upper rectilinear edge and onto the hemispherical base along its circular edge. Generally, the tube sheet is made of carbon steel coated with stainless steel, the hemispherical base is made of cast or forged steel, and the partition plate is made of solid stainless steel. The welds are consequently difficult to produce and test, the welding causes deformations which make bringing the base up to the tube sheet tricky when the water box is assembled, and the residual stresses induced cannot always be eliminated by relieving the tension. Until now, these various problems have been only partly solved, for example by fixing a connecting piece to the lower face of the tube sheet which facilitates welding of the partition plate.
However, in addition, because of the differential deformations of the tube sheet and the hemispherical base, calculation of the mechanical behavior of the partition plate connected rigidly to the two parts is very tricky.
Moreover, the division of the hemispherical base into two chambers presents another disadvantage. In practice, it is sometimes necessary to gain access to the interior of the water box for inspection and maintenance operations. It is essential in this instance to entirely empty the water box of the contaminated water it contains. To accomplish this, in each chamber, at the lower part of the hemispherical base, it is necessary to pierce drain orifices which open into the pipe for the inlet or outlet of the primary fluid. But to completely empty the space bounded by the man-hole closing plate and its cylindrical inner face positioned in the depth of the hemispherical base, other drain orifices in the inner wall of the man-hole must also be pierced.
These various drain orifices are difficult to machine and always run the risk of being blocked by deposits. They can also slightly weaken the wall of the hemispherical base just where the pipes start.